The Siemens method is known as a method for manufacturing polycrystalline silicon which becomes a raw material of single crystal silicon for use in manufacturing a semiconductor device or of silicon for use in manufacturing a solar cell. The Siemens method is a method of bringing a source gas containing chlorosilane into contact with a heated silicon core wire and thereby growing polycrystalline silicon on the surface of this silicon core wire in the vapor phase by using a CVD (Chemical Vapor Deposition) method.
When polycrystalline silicon is grown according to the Siemens method, silicon core wires are assembled into a guard frame shape consisting of the two wires in perpendicular directions and one wire in horizontal direction in a reactor of a vapor-phase growth apparatus, and both ends of the silicon core wire constituting this guard frame shape are fixed on a pair of metal electrodes which are arranged on a base plate through a pair of core wire holders.
Next, a source gas such as a mixture gas of trichlorosilane and hydrogen is supplied into the reactor through a gas nozzle, while the silicon core wire is heated in a temperature range of 900° C. or higher and 1,200° C. or lower in a hydrogen atmosphere by an electric conduction of an electric current passed from the metal electrode, then crystals of silicon grow on the silicon core wire, and polycrystalline silicon with a desired diameter is formed into an inverted U-shape.
When single crystal silicon is produced with an FZ (Floating Zone) method using polycrystalline silicon which has been grown in the vapor phase according to the Siemens method, polycrystalline silicon with the above described inverted U-shape is subjected to the treatments of: preparing a polycrystalline silicon rod with a desired length through a cutting process of cutting both ends of the polycrystalline silicon (cutting process); grinding the perimeter of this polycrystalline silicon rod so that the diameter becomes equal in a longitudinal direction and the surface becomes uniform (cylinder grinding process); further machining one end of this polycrystalline silicon rod to sharpen the end (tip machining process); and finally etching the surface of the polycrystalline silicon rod to remove impurities and distortion (etching process).
In such a polycrystalline silicon rod, cracking (crack) tends to be easily formed in inside and outside parts, in the vapor-phase growth process or a cooling stage after the growth, along with a large diameter tendency in recent years.
When the cracking is formed in the inside and outside parts of the polycrystalline silicon rod, the rod may be broken in the above described cutting process, cylinder grinding process, tip machining process, or etching process. Alternatively, in the worst case, the polycrystalline silicon rod may be cracked in a process of growing a single crystal silicon ingot with an FZ method. If the polycrystalline silicon rod is cracked in any of these processes, not only the process operation conducted before the time becomes useless, but also even the equipment which is used in the process may be damaged.
In addition, when the polycrystalline silicon rod is used as a supplemental charge material or a recharge material in the process of growing the single crystal silicon ingot with the CZ (Czochralski) method, if there is the cracking in the inside and outside parts of the polycrystalline silicon rod, the polycrystalline silicon rod may be ruptured initiating from the cracking while being machined so as to become usable in a rod shape as it is or while being lowered into a crucible in a CZ furnace heated to a high temperature, and may fall into the CZ furnace to scatter a silicon melt or destroy the crucible.
Here, the supplemental charge means to increase the amount of the melt in the crucible by making the polycrystalline silicon rod which has been hung above the crucible gradually melt into the melt, after a silicon lump which has been charged into the crucible has been melt. In addition, the recharge means to increase the amount of the melt in the crucible by making the polycrystalline silicon rod which has been hung above the crucible gradually melt into a remaining melt, after the CZ crystal has been pulled up.
Conventionally, various techniques are proposed in order to detect the cracking of the inside and outside parts of polycrystalline silicon. For instance, Japanese Patent Laid-Open No. 2001-21543 (Patent Literature 1) discloses a flaw detection method of placing a polycrystalline silicon lump in water or another liquid, transmitting and receiving a sound wave of 0.5 to 10 MHz while making a probe scan the polycrystalline silicon lump, in an upside of the polycrystalline silicon lump, and displaying an abnormal portion right under the probe on a two-dimensional plane.
In addition, Japanese Patent Laid-Open No. 2007-218638 (Patent Literature 2) discloses a crack inspection method of comparing image data provided by an infrared transmission light through a polycrystalline silicon wafer, with image data provided by an infrared reflection light therefrom, obtaining a difference between brightnesses or luminances of each pixel corresponding to the same position, and determining whether there is a crack in the inside and outside parts of the polycrystalline silicon wafer or not.